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PostPosted: Tue Jan 02, 2018 4:35 pm 
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Bottle Rocket
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Hey guys I've got a 1985 black stripe Mesa Boogie Mark III

The amp was working fine but since the electrolytics are 33 years old I went ahead and ordered a set from Mesa Boogie.

Yesterday I changed two of the big 220uf 300v filter caps.
I also changed one of the three 30uf500v preamp caps. The reason I didn't change them all as because I didn't have enough capacitors and I had to order a few more from Mesa Boogie.

I also installed a new set of power tubes and preamp tubes that I ordered directly from Mesa Boogie.

Anyway I plugged in the amp turned it on and waited for about 5 minutes then flipped on the standby. I heard the amp make a crackling sound for about 2 seconds and then everything started working fine and I played it for about 10 minutes. Then all of a sudden the fuse blew and I lost power.

Could this be due to mixing new capacitors with the old capacitors? My soldering skills are very good and I don't think I made a mistake soldering.

Here are a couple pictures showing the Caps that I replaced, pic is before I replaced them. I made sure the polarity was correct when I replaced them.

In the picture I've replaced the 2 big ones on the right and one of the ones on the left. (There's two more big ones underneath the two on the right I still have to replace those)

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PostPosted: Wed Jan 03, 2018 1:54 pm 
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Mark II
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I don't know how much help I can be, but the only time I've blown a fuse on my MKIV was when one of the power tubes crapped out.
I know you said you replaced the tubes, but I would check the power tubes. The last time I cooked a tube, it took me almost 15 minutes to find the scorch mark.

You're probably good at electronics, but have you checked the caps with a multimeter? I don't know much (if anything) about circuits but it might be worth a shot.

https://www.electronicshub.org/how-to-test-a-capacitor/

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PostPosted: Thu Jan 04, 2018 12:45 am 
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Bottle Rocket
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I visually inspected the tubes and they look fine and I don't see any Scorch marks on the tube sockets. But I think you're right from everything I've researched it sounds like this only happens due to a bad power tube. I'm going to put the app back together and put in a new tube and see what happens.


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PostPosted: Sun Jan 07, 2018 4:54 am 
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Mark II

Joined: Sat Aug 05, 2006 11:15 pm
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if its a screen resistor failure, the next tube will blow too!

plus capacitors are supposed to be burned in before you use the amplifier? with some kind of dimmer switch in between power socket and the amplifier plug

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PostPosted: Sun Jan 07, 2018 10:21 am 
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Bottle Rocket
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I asked Mesa Boogie about burning in the capacitors and they said no I did not have to do that.

I think that really applies to Old amps that have been sitting for a long time.


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PostPosted: Sun Jan 07, 2018 2:02 pm 
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Single Recto

Joined: Sun Feb 19, 2006 11:43 am
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Location: NYC
Although its not absolutely necessary to "pre-form" electros,its not a bad idea,I do it with all new caps.But thats not likely the problem.It is possible that because you changed the main supply caps the voltage downstream to those old caps is stiffer and after a while the extra voltage is shorting one or more of the older/weaker caps.Could be a number of other things as well,including tubes,something wrong in the caps you changed etc.Would be more useful to see a pic after you changed the caps,might show something useful.


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PostPosted: Sun Jan 07, 2018 2:29 pm 
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Bottle Rocket
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The blue ones are replace in the amp. Here are the new ones :).

Turns out the problem was a bad el34 power tubes even though it was a brand new one ordered from Mesa.

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PostPosted: Sun Jan 07, 2018 4:28 pm 
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Single Recto

Joined: Sun Feb 19, 2006 11:43 am
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It happens.Especially with these current production tubes.Glad you got it sorted out.Looking at the "new" pic,I see a loose red wire hanging by those 220uf's.


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PostPosted: Sun Jan 07, 2018 6:42 pm 
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Bottle Rocket
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Damn your good man.... yeah that loose red wire was to the standby switch. I accidentally broke it loose when I was lifting off that small board to put the big capacitors underneath it. I soldered it back into the hole and everything's good to go.


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